Machines such as wheeled compactors, loaders, trucks, and other mobile equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more ground engaging traction devices. Such machines usually include mechanical power-shift transmissions that are connected to the engine by way of a mechanical or hydraulic torque converter, which is intended to cushion the connection between the engine and the transmission. When the engine is disconnected from the transmission via the torque converter, for example when the operator desires to terminate acceleration of the machine, naturally occurring friction within the torque converter causes a gradual slowing down or coasting of the machine.
Consequently, operators have grown accustomed to machine coasting upon terminating a request for machine acceleration (e.g., upon releasing an acceleration pedal or lever). For example, a compactor operator may know that the speed of a conventional compactor may slow down by a specific amount over a certain period of time or within a certain distance and, accordingly, control the machine with expectations of the coasting. Therefore, operators have become comfortable using coasting to complete specific tasks.
Recently, however, conventional power-shift transmissions are being replaced by hydrostatic transmissions (hystats). A hystat provides an infinitely variable torque-to-speed output ratio within its overall range through the pairing of a variable displacement pump and a fixed- or variable-displacement motor. Although a hystat is known to have higher efficiency and extended functionality when compared with a power-shift transmission, it does not behave in the manner discussed above with respect to coasting. Specifically, when an operator releases the acceleration pedal or throttle, displacements of the pump and/or motor are immediately neutralized and the machine either continues traveling at about the same speed for an extended period of time (when both the pump and motor are neutralized) or quickly stops (when only one of the pump and motor are neutralized). Therefore, operators must modulate the acceleration lever or pedal to move through intermediate displacement positions in a gradual manner in order to travel at an increasingly slower speed. This can prove very difficult in rough terrain common to many worksites, especially for unskilled operators, and can cause undue operator fatigue. Accordingly, there is a need to gradually slow the speed of a hystat-equipped machine in an alternative manner.
One such method of machine control is described in U.S. Pat. No. 6,341,488 (the '488 patent) of Shimizu et al. that issued on Jan. 29, 2002. The '488 patent describes a hystat transmission having a variable displacement pump paired together with a fixed displacement motor. In order to inhibit an abrupt stop of a vehicle when an operator releases a control arm, the pump is provided with a piston that generates a friction force against a contact plate of the pump. The friction force resists rotation of the pump's swashplate toward the neutral position such that a return spring returns the swashplate to neutral in a smooth manner.
Although the system of the '488 patent may allow for smooth stopping of a hystat-equipped vehicle, it may still be problematic. In particular, the system of the '488 patent may not allow for coasting of the vehicle for extended periods of time. In addition, the system of the '448 patent may lack applicability to a hystat where displacements of both the pump and motor may benefit from coordinated control.
The system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.